Posted
by
Soulskill
on Wednesday May 30, 2012 @03:27PM
from the you-wouldn't-like-me-when-i'm-angry dept.

scibri writes "Sagittarius A*, the dormant supermassive black hole that lies at the center of our galaxy, was much more active not that long ago. Astronomers using the Fermi Gamma-ray Space Telescope have picked up some faint gamma-ray signals that suggest Sagittarius A* was emitting a pair of powerful gamma-ray jets like other galactic black holes as recently as 20,000 years ago (arXiv paper). If our black hole was more active in the past, it could explain why Sagittarius A* seems to be growing about 1,000 times too slowly for it to have reached its current mass of about four million solar masses since the Galaxy formed about 13.2 billion years ago."

From the view of an outside observer, the larger (greater mass) black hole will appear to draw the smaller one in. In reality, they're drawn to each other, and eventually merge, like two drops of water meeting on a window pane. The singularities become one. Simulations suggest that the merger will radiate massive quantities of energy in the form of gravity waves, a major ringing of the bell.

The ringing quickly dies down, and the only evidence of the merger being 1) the changed direction of movement relative

Lack of gas and dust streaming in. The disk + torus the infalling gas produces while accreting produces all the radiation we see from black holes in active galactic nuclei (AGN). Another side effect are the jets that you can see in radio frequencies (although not in all AGN.

I'd like to know where this black hole came from. Was there some random star floating through space, which died, and then it started gobbling up everything? Including our galaxy (which will eventually fall in). Or maybe the superblackhole was a previous galaxy from ~25 billion years ago that fell into itself?

I'd like to know where this black hole came from. Was there some random star floating through space, which died, and then it started gobbling up everything? Including our galaxy (which will eventually fall in). Or maybe the superblackhole was a previous galaxy from ~25 billion years ago that fell into itself?

To my knowledge it is currently unknown how those massive black holes (millions of solar masses) form originally. We know they form very early in the universe (1Gyrs after the big bang, our universe is ~14Gyrs old). Do they come from many stars? Were stars in those early times extremely massive? Is there some way of growing black holes very fast?Those are open questions in Astrophysics... you are welcome to join in:)

The singularity itself? A teaspoon of singularities would have infinite weight. Maybe you mean everything inside the event horizon? In that case calculate the Schwarzschild radius (2Gm/c^2) of 4 million solar masses, then get the density [4 million solar masses/(4/3 pi r^3)] and multiply by the volume of a teaspoon. I think the density of everything inside the event horizon for that big of a black hole is actually pretty low.

I was doing the calculations originally suggested by Colonel Korn. Presumably the Schwarzschild radius isn't completely accurate here, and the "four million suns" mass figure is of course not very precise.

Still, the answer I was replying to was off by around 33 orders of magnitude (which it got modded up for...) whereas mine was only off by a factor of 5--and I'm not sure how precise WA's figure is.

You've touched on one of astrophysics' proverbial elephants in the room.

There is no physics-based reason to believe there is a "singularity" at the center of a black hole, only a place where the math breaks down to a division-by-zero. In reality, some physical constraint would prevent further collapse, in the same way that degeneracy pressure halts the collapse of less massive stars.

I haven't read Susskind yet, but I have read General Relativity by Dirac.

According to Dirac's, the singularity will never actually occur because of time and space dilation. The stellar matter will accelerate towards the singularity but never actually reach it. And never is an appropriate term since according to Hawking the black holes have a finite lifetime and will eventually evaporate according to Quantum Mechanics.

Statistically speaking almost no-one knows this. At its extremes these limits are often interpreted wrongly. - Mostly by people that have never built anything.In most "physical" problems, an answer approaching 0 means you are about to waste your time. An answer of infinity is supposed to let you know you have NO idea of WTF you are doing.Everything in between is actually pretty interesting. Mathematical models are things of inherent beauty and most of it in theoretical physics is above my pay grade.

well, since the gamma rays are practically pointing directly perpendicular to the rotation of the galaxy, they wont come anywhere near us. Even if it were pointing directly at us, it would take them at least 26,000 years to reach us. Granted, we wouldn't know they were coming until we were fried.

The jets should be perpendicular to the accretion disk which should mostly be co-planar with the disk of the galaxy. So, the jets will not point at us. If some oddball star or mass or whatever that is in a highly out of galactic plane orbit gets sucked in, we should still be ok as there is quite a lot of dust between us and Sag A*.

Speaking of incomplete data - No, we have evidence that gamma rays leave a vapour trail in a cloud chamber since that's how we detect them in an atom smasher. A few people have taken this fact and speculated that gamma rays affect the climate by seeding clouds, there are even a couple of books about the idea. Only problem is, their speculation does not not fit everyone else's observations. However you will find the 'Iris theory' presented as fact in the opinion pages of the wall street journal. Why? - Because on top of their complete lack of evidence, the people pushing this idea just happen to think it 'proves' Earth's climate is self regulating so we don't have to worry about regulating our emmissions.

Chuck Norris got in its face. He told it to quit sucking so much, or he'd tear it a new asshole, reach inside, grab its singularity and turn it inside out. That's why we now have a quiet black hole in our galaxy.

Is anyone else disturbed that such an incredibly major change happened only 20,000 years ago?

This could be worse than an ice age.

No. If, 20,000 years ago, it was much more active, it proves living in a galaxy with an active nucleus is not a problem. What it means is, if it becomes more active again, we don't really have anything to worry about -- we've been living with the "problem" for most of five billion years and gotten along just fine...

Unless there is some unknown thing about the radiation stopping that allowed civilization to develop.Like a lower rate of mutations allowing humanity to become genetically stable.Sounds like a good Sci-Fi premise.

One example:http://www.etheric.com/LaViolette/Predict.html [etheric.com] "Subsequent concurrence (1998): In 1988, when presented with Dr. LaViolette's Galactic explosion hypothesis, astronomer Mark Morris dismissed the idea as having no merit. However, in 1998 after ten years of observation, Morris was quoted as saying that the center of our Galaxy explodes about every 10,000 years with these events each lasting 100 years or so."

It was explained in my astrophysics class that when a black hole reaches a certain mass that whole stars pass inside the event horizon before being torn up by tidal force.
Then the singularity no longer has a big accretion disk and the radiation emitted by infalling matter is trapped within the event horizon.
So it goes quiet.

It was explained in my astrophysics class that when a black hole reaches a certain mass that whole stars pass inside the event horizon before being torn up by tidal force.

Then the singularity no longer has a big accretion disk and the radiation emitted by infalling matter is trapped within the event horizon.

So it goes quiet.

Who was your lecturer? Pick a mass, any reasonable mass for a black hole, and calculate it's Schwarzschild radius. Now add the diameter of a star, any star, to that radius and calculate the gravity field at that distance from the center of the black hole. Do you notice the difference? That tidal force will tear any star apart. It will tear iron apart.

Furthermore, it is very unreasonable to assume that the only object orbiting the black hole is the single ingested star. Most (all?) black holes have large qua

Just a followup: let us assume the largest black hole that we can imagine, ingesting the smallest star that we can imagine, in the interest of minimising the tidal forces on the star. In this extreme hypothetical case of no tidal force on the star until it contacts the event horizon, you will agree that the star will have also drag down with it minimal material from the accretion disk, and certainly not all of it. Now imagine what a star passing through the remaining material does to the distribution of tha

The Schwarzchild radius is proportional to the mass M. The tidal effect is proportional to the derivative of the gravitational field, which is proportional to M / R^3. Setting R to be the Schwarzchild radius, to measure the tidal effect at this point, we find that it is proportional to 1 / M^2. So the more massive a black hole is, the smaller the tidal effect at its event horizon - and for a sufficient black hole mass, the tidal effect must be insufficient to break up a star.

The Schwarzchild radius is proportional to the mass M. The tidal effect is proportional to the derivative of the gravitational field, which is proportional to M / R^3. Setting R to be the Schwarzchild radius, to measure the tidal effect at this point, we find that it is proportional to 1 / M^2. So the more massive a black hole is, the smaller the tidal effect at its event horizon - and for a sufficient black hole mass, the tidal effect must be insufficient to break up a star.

Calculating the mass at which this happens is left as an exercise for the reader.;-)

Thanks. I address this in my followup post, posted as a reply to self. And I also left the "insufficient tidal effect" BH mass as an exercise for the reader, for any arbitrary definition of insufficient!

The most massive black holes ever observed are over a billion solar masses. At 90 million solar masses, the sun could hold together at least with respect to gravity vs. tidal force.
If you were observing this happen, you would see the sun getting dimmer and redder as it fell toward the event horizon, and at some point I think it would just go dark. You wouldn't see a flash or flare because a big piece of matter just disappeared from the observable universe without making a splash.